It is well known that turbines are provided to extract energy from the hot gas stream as it impinges on the turbine blades which in turn cause a rotary action of an associated rotary apparatus. The blades are in the form of air foils and manufactured from materials capable of withstanding extreme temperatures. On the other hand, their mounting is designed to withstand high mechanical loads and stresses as against the high temperature requirement of the blades. For this reason, it is important to protect the mounting or shank portions of the blades from the direct impact of the high temperatures of the hot gas stream. Therefore, the blade and vane elements of the turbine are provided with platforms which axially combine to define a boundary for the hot gas stream isolating the mounting shank portions from the hot gas stream.
Such protective attempt is equally important throughout the rotor cavity. However, it becomes more pronounced in the interstage region of the high pressure portion of turbine where the boundary of the expanding hot gases comes close to temperature sensitive areas of the rotor cavity, such as the forward and aft cavities bounded by the disk post for the stage one blade wheel, the platform for the stage two stationary nozzle assembly and by the disc post of the stage two blade wheel.
According to present practice labyrinth-type seals are used between the forward and aft cavities. Such seals are well known in the art and include a plurality of circumferential teeth which are contiguous with a circumferential sealing surface made from a high temperature resistant abradable material or other deformable materials to form the sealing surface with which the labyrinth teeth coact and, due to the deformability of the honeycomb material, the sealing surface becomes deformed without injury to the teeth thereby establishing a minimum clearance required under the operating conditions.
When such labyrinth seal is installed in the high pressure interstage region of the high pressure or HP turbine between the forward and aft cavities as can be seen from the detailed description hereinafter, during operation cooling air passes through the HP stage two nozzle and purges the forward cavity behind the stage one disk. This air then leaks through the labyrinth seal to purge the aft cavity in front of the stage two disk post. With such arrangement, stage two disk creep has been found due to the temperature rise in the cooling air as it passes through the labyrinth seal and, for some operating conditions, inflow of the hot gas stream into the aft cavity due to insufficient purge flow. In order to remedy the above-noted deficiency, axial slots were incorporated into the honeycomb portion of the interstage seal to supply additional cooling air directly to the aft cavity and thus reduce the net aft cavity air temperature. It has been found, however, that the air stream leaving the axial slots requires energy input to be accelerated to the rotor speed, increasing the temperature of the air relative to the stage two rotor. Therefore, the efficiency of such a system remained below expectations.